Hitherto, inorganic photoconductive materials such as selenium, cadmium sulfide, zinc oxide or amorphous silicon have been used as electrophotographic photoconductive materials. The inorganic photoconductive materials advantageously have good electrophotographic characteristics which include good photoconductivity, charge-acceptability in the dark and insulating property. On the other hand, however, they have various disadvantage. For instance, the manufacture cost is high, they often cause environmental pollution, they lack flexibility and they are not sufficiently resistant to heat and mechanical shock.
Recently, in order to overcome the drawbacks of the inorganic photoconductive materials, electrophotographic photoreceptors made of organic photoconductive materials have widely been studied and various kinds of such photoreceptors have been proposed.
In organic electrophotographic photoreceptors of the kind, the leading style is a laminate type photoreceptor having two layers of a charge-generating layer which generates a charge carrier by exposure and a charge-transporting layer which transports the charge carrier generated.
The charge-generating layer is basically composed of a charge-generating substance which absorbs light to generate a charge carrier and a resin binder. The charge-generating substance is required to be uniformly dispersed in the layer. Non-uniform dispersion of the substance would often cause defects of white spots or black spots on the image formed. Therefore, selection of the binder is important. In addition, in electrophotographic process, high photosensitivity and small potential fluctuation in repeated use are required, and the charge carrier as generated in the charge-generating layer must be introduced efficiently into the charge-transporting layer. The resin binder for dispersing the charge-generating substance therein has an extremely large influence on the mobility of the charge carrier generated, with respect to the chemical structure, molecular weight and purity of the resin binder itself.
In the photoreceptor of such a type that the charge-generating layer is first formed on an electro conductive support and then the charge-transporting layer is formed over the charge-generating layer, when the resin binder in the charge-generating layer is soluble in the coating solvent for the charge-transporting layer, the previously formed charge-generating layer would be dissolved out during formation of the charge-transporting layer to cause intermixture in the interlayer between the charge-generating layer and the charge-transporting layer. As a result, the sensitivity is lowered, the residual potential is elevated and the electrophotgraphic characteristics are lowered. Such are troublesome problems. In addition, the image to be obtained also has problematic drawbacks of white spots, fog and image blur.
As the resin binder to be used in the charge-generating layer, polyester resins are used in combination with azo dyes (JP-A-54-22834). (The term "JP-A" as used herein means an "unexamined published Japanese patent application.) Further, use of hydropropyl cellulose resins (JP-A-57-169754), fatty acid cellulose ester resins (JP-A-58-166353), acrylic resins (JP-A-58-192040), polyvinyl butyral resins in combination with polyamide-subbing layer (JP-A-58-30757), linear polyester resins in combination with copolymerized polyamide-subbing layer (JP-A-58-93739), or phenoxy resins, polyvinyl formal resins or ethyl cellulose resins in combintion with alcohol-soluble nylon-subbing layer (JP-A-60-196766, JP-A-60-202448, and JP-A-60-202449) has heretofore been reported.
However, even when the said resin binders are used in the charge-generating layer, the sensitivity, image uniformity and solvent-resistance are still insufficient. Under the situation, further improvement of the resin binders for the charge-generating layer are being desired.
On the other hand, various measures have been effected so as to impart high charging property, high sensitivity and low residual potential to the laminate-type photoreceptor. As one of such measures, it is known to provide a subbing layer between the electroconductive support and the photosensitive layer so as to prevent introduction of the electric charge from the electroconductive support to the photo-sensitive layer.
In addition, defects, stains or scratches, if any, on the surface of the electroconductive support would cause drawbacks of coating failure, spots or unevenness in formation of the photo-sensitive layer, which would thereby result in formation of non-uniform image with white spots, black spots and fog. In order to overcome such non-uniform image, it is also known to be effective to provide the subbing layer.
As the resins to be used for the subbing layer, nitrocellulose resins (JP-A-48-3544), polyvinyl butyral resins (JP-A-50-28837 and JP-A-59-36258), polyvinyl alcohol resins (JP-A-52-100240), alcohol-soluble nylon resins (JP-A-52-25638), polyvinyl formal resins (JP-A-57-90639), water-soluble polyvinyl butyral resins (JP-A-58-106549}, blend of polyvinyl butyral resin and phenol resin (JP-A-59-36259), as well as resins of polyvinyl methyl ether, polyvinyl imidazole, polyethyleneoxide, polyurethane, polyamide, melamine, polyvinyl pyrrolidone, cellulose ester, casein and gelatin are known.
Further, it is also known to add electron donating substances, charge-accepting substances, fine grains of metal oxides or quaternary ammonium salt compounds to the subbing layer to control the electric resistance of the layer so as to improve the electrophotographic characteristics of the photoreceptor (JP-A60-140357, JP-A-59-170846 and JP-A-59-84257).
However, in the photoreceptor of the type having such subbing layer, the film thickness of the subbing layer is required to be made thick so as to enrich the charging property of the photoreceptor, but the thick subbing layer film causes a problem of increasing the residual potential after exposure. In addition, the subbing layer is often affected by the influence of water in air, so that the photoreceptor having such subbing layer is to have high electric charge and high residual potential under the condition of low temperature and low humidity but to have low electric charge and low residual potential under the condition of high temperature and high humidity. Thus the photoreceptor of the kind has a troublesome problem that the electrophotographic characteristic fluctuates in accordance with the ambient circumstance.
Moreover, the photoreceptor having a subbing layer has another problem that the residual potential increases in repeated use to give background fog in the image formed.